The great majority of soap bars on the market are made by extrusion of a soap base formulation. These soap bases are generally opaque due to two factors, first, the crystallinity of the chemical composition of the soap base, and second, the presence of pigmentary titanium dioxide. When a translucent soap is made, the base formulation does not contain the added pigmentary titanium dioxide and, further, it is modified by the addition of small amounts of polyhydroxy compounds so as to increase the transparency.
Pearlescent pigments have been used in extruded soap bars to effect certain appearance effects and a translucent soap base is desirable for that purpose. Japanese patent publication 54-26807 describes extruded soap bars having a pearly gloss and a wood grain appearance, but not with a bright color.
U.S. Pat. No. 4,584,126 is concerned with transparent soaps of a particular composition and processes for manufacture thereof. A process is described for making the translucent soap bars pearlescent in appearance by the incorporation in the formulation of small amounts of mica or of pearlescent pigments. However, this approach is not particularly successful for several reasons. First, mica is too close in refractive index to the soap base to impart significant pearlescent reflectivity. Second, the soap base physical structure is such as to inhibit the orientation of the platey pigment particles into parallel layers. Moreover, there is no suggestion of the use of pearlescent pigments in extruded soap bars to achieve various attractive color effects, as distinct from pearlescent effects alone.
Copending patent application, Ser. No. 617,544, filed Nov. 26, 1990, describes extruded soap bars having an improved wood grain appearance. Such extruded soap bars are not brightly colored.
While extruded soap bars with color are common and numerous, they are dull or matte in appearance and very limited in the choice of colors. These unattractive features are due to the light scattering or coverage character of the conventional colorant pigments. Also, because most organic colorants are not stable in the alkaline medium of extruded soap base formulations, the choice of colorants is very limited.
Further, when dyes are used in extruded soap bar formulations, they can bleed badly when the soap is wet in use. In fact, most pigmentary colorants also bleed badly in wet use.
The presence of a pearlescent pigment in an extruded soap bar has, as noted above, been asserted to render the soap bar pearly in appearance. However, a pearlescent pigment incorporated in a transparent, or translucent, medium must have its flat and platey pigment particles oriented into parallel layers so that the incident light may be reflected in a specular manner to yield the pearly appearance. This optical behavior has been described many times, for example, in Nacreous Pigments, L. M. Greenstein, Encyclopedia of Polymer Science and Technology, Vol. 10, 1969, John Wiley & Sons, Inc., pp. 193-215.
It is, in fact, very difficult to achieve this high degree of two-dimensional orientation in extruded soap bars. The reason is at least in part due to the structure of the soaps, which had been found many years ago to be crystalline and subsequently it was recognized that soap bases consist largely of liquid crystals. Using extrusion techniques on soap base formulations containing pearlescent pigments, it is possible to attain some limited orientation of the platey pigment particles, usually one-dimensional in the direction of the extrusion axis, but not two-dimensional for any distance. A uniform pearly appearance is not readily attained, except over small areas of 1-5 mm in distance.
It has now been found that the use of certain pearlescent pigments overcome the objections of conventional colorants presented above. Very significantly, these pearlescent pigments scatter light minimally, or have low coverage. As a consequence, the colored pearlescent pigment imparts color brightness to an extruded soap bar. Even with the limited orientation of the pearlescent pigment particles, a brightness and depth is obtained, which is not displayed by soap bars containing other types of pigments. With colored pearlescent pigments, this becomes a brightness of color.
It is an object of this invention to produce extruded soap bars having a brightness and clarity of color. This and other objects of the invention will become apparent to those skilled in the art from the following description.